The present invention generally relates to noise reduction circuits, and more particularly to a noise reduction circuit for use in a video signal recording and/or reproducing apparatus such as a video tape recorder in order to eliminate noise components.
Video tape recorders using a so called color-under recording system are widely used. In such a video tape recorder, a luminance signal is frequency modulated and recorded on a magnetic tape together with a frequency converted carrier chrominance signal having a frequency range lower than that of the luminance signal in accordance with the frequency division multiplexing technique. Such a video tape recorder has a tendency that the signal-to-noise ratio (S/N ratio) deteriorates with increasing frequency due to the increase of noise components having a so called triangular spectrum. In order to avoid this problem, the video tape recorder uses a noise reduction circuit which employs a preemphasis to the luminance signal to be recorded prior to the frequency modulation. In other words, a high frequency component of the luminance signal is extracted and added to the original luminance signal after suitable processing. As a result of the preemphasis, the high frequency component of the luminance signal is enhanced. At the time of reproduction, the same noise reduction circuit is used to perform a deemphasis, which is complementary to the preemphasis, to the demodulated luminance signal to complement the preemphasis. In the description hereinafter, the term emphasis will be used to include both preemphasis at the time of recording and deemphasis at the time of reproduction.
In recent years, there is a need to increase the degree of emphasis mainly as a result of the broadened frequency range of the video tape recorder, for example, from the conventional 3 MHz range to 5 MHz range. Generally, the increase in the degree of emphasis certainly increases the effect of noise reduction. However, if the degree of emphasis applied was excessive, the carrier frequency of the frequency modulated luminance signal would deviate too much. This would result in the reproduced picture so called inverted white peak. In other words, when the degree of preemphasis of the high frequency component of the luminance signal becomes excessive as a result of the preemphasis, the side band components of the frequency modulated luminance signal interferes with the frequency converted carrier chrominance signal to such an extent that the reproduced color signal is deteriorated. In order to prevent the excessive frequency modulation of the luminance signal, a white clip circuit is usually used to eliminate from the frequency modulated luminance signal the frequency component deviating to such an extent that it interferes with the frequency converted carrier chrominance signal. However, the use of such a white clip circuit is associated with a possibility of causing a poor picture quality due to the fact that the circuit may cut too much of deviated frequency components.
In order to avoid this problem, the conventional noise reduction circuit uses a limiter circuit which limits the amplitude of the high frequency component of the luminance signal at a predetermined level. Such a limiter circuit generally has an input versus output amplitude characteristic which changes the slope of the characteristic curve stepwise in two stages as the amplitude of the input signal increases.
In the noise reduction circuit using the limiter circuit having such an amplitue characteristic which changes stepwise in only two stages, the slope of the characteristic curve changes sharply when the gain for the input signal having small amplitude is set to a relatively large value. Such a sharp change in the slope of the characteristic curve leads to a difficulty in recovering the original signal at the time of deemphasis. Because of this, the reproduced picture is deteriorated.
Further, such conventional noise reduction circuit has a problem that the emphasis applied to the input luminance signal is virtually the same for a case in which the level of the high frequency components contained in the input luminance signal is small and for a case in which the level of the high frequency components contained in the input luminance signal is intermediate. For example, the effect of preemphasis for the input signal having a level of-20 dB referred to the reference level of 0.4 Vp-p is virtually the same as the effect of preemphasis for the input signal having a level of-30 dB.
Furthermore, the conventional noise reduction circuit uses a feedback loop for appropriately shaping the waveform of the output signal. However, the use of such feedback loop tends to cause oscillation when the setting of the amount of feedback is not appropriate.
Furthermore, the conventional noise reduction circuit has a problem in that a horizontally streaking noise appears on the screen in the reproduced picture responsive to incoming of a noise impulse. In order to avoid this horizontally streaking laterally spread noise associated with the noise impulse, one has to admit, as a trade-off, a residual random noise visible for relatively long periods, which is of course a disadvantage.